Charging phenomena in PEEM imaging and spectroscopy.

Spectromicroscopy with the imaging technique of X-ray photoelectron emission microscopy (X-PEEM) is a microchemical analytical tool installed in many synchrotron radiation laboratories, and which is finding application in diverse fields of research. The method of sample analysis, X-ray absorption spectroscopy, does not encounter the same problems as X-ray photoemission spectroscopy when sample charging occurs, hence even good insulators may often be analyzed without any apparent artifacts in images or spectra. We show, however, that charging effects cannot be neglected. We model the effect of surface charge formation on the secondary electron yield from uniform samples to demonstrate that surface charge primarily reduces the yield of electrons which may contribute to the detected signal. We illustrate that on non-uniform insulating samples, localized centers of charge may substantially affect microscope imaging and resolution as the electrostatic field close to the surface is distorted. Finally, in certain circumstances non-uniform surface charge may lead to unexpected lineshapes in X-ray absorption spectra causing, in some extreme cases, negative spectra. These negative spectra are explained, and several strategies are reviewed to minimize the impact of sample charging when analyzing poorly conducting samples of any nature.